[TJ's GMC]

Originally Posted by Chuck78

Wow... quite the load there TJ!
I'd like to toy with the idea of adding a SMALL 4 bbl carb (Carter AFB or maybe Rochester Quadrajet, maybe Holley Street Avenger) to the 351, but on a 351, 600 cfm would really be the useable limit. These engines were intended for low rpm, they have no harmonic balancer even. Doing mods than are only beneficial at 4000+ rpm will tear up these engines.

I think a 351 may be able to get even better fuel mileage if I were to run a small 4 bbl on it.

600 cfm on a 305 is just overkill. You'd likely get better low end response with a smaller carb. Having your cam reground to add about .020" lift and running custom higher compression pistons to be able to run magnum heads on a 305 (otherwise the big chambers will drop your cr below useable), that would require a little more carburetion, and make great power.

The thing that always gets me in the hot rod automobile community vs vintage motorcycle community, is that the bigger automotive engines mask mixture issues more than a smaller bike engine, so people never bother changing out carb jetting to fine tune the engines!

I have a wide and o2 sensor and AFR gauge/controller tailpipe sniffer setup nowadays. I have yet to put it on the GMC, but will when I run the 351C with the Holley 500 conversion. I hope to add an X pipe to the exhaust at some point, and maybe get some custom headers made. Dual exhaust is overrated, that's why H pipes became common and now the superior X pipe dual setup is popular. Same reason everyone started making 4 into 1 headers for 4 cylinder race bikes following Pops Yoshimura's lead...

Thanks for the info Chuck! The overall performance was greatly increased, even with an over kill carb. The motor has no issues burning the fuel with stock points and a pertronix coil. I've lost a tad bit of low end, but not enough to where it's really noticeable. When towing I can shift at 2700 RPM and still pull through the gears whereas before I had to shift over 3000. That load I towed there I never shifted over 3000 rpm with! I may try getting a 500 edelbrock and jetting it down a bit, or a quadra bog. But I just recently pulled some plugs again and they still look really good! Gas mileage I got a rough estimate of 10. lol But I have been doing a lot of towing and stopping and going. No real highway driving. Part of that problem to would be the 4.11 gearing and 29 inch tires. I plan on 31's or 32's in the near future as I finally got a set of 16 inch rims. Even the inside of the exhaust is not as black as before. I've been playing with the idea of getting that MSD fuel injection unit. lol